Carbon Fiber Mechanical Properties: Data Ref

Ask an engineer who’s never worked with composites what carbon fiber’s tensile strength is, and they’ll give you a single number—maybe 3,500 MPa, maybe 5,000, maybe “stronger than steel.” All three answers are both true and completely misleading. Carbon fiber doesn’t have a tensile strength. It has a tensile strength in the fiber direction, a different tensile strength in the transverse direction, and a shear strength that’s independent of both. The number on the datasheet—the 4,900 MPa for T1000G, the 3,530 MPa for T700S—is a unidirectional test result measured along the fibers. Rotate the load 90 degrees and the strength drops by a factor of fifty. This directional behavior is the single most important thing to understand about carbon fiber mechanical properties, and it’s the thing that isotropic-material intuition gets wrong every time. This article is a reference for the mechanical properties that matter in design: tensile, compressive, flexural, and fatigue performance of carbon fiber composites, with data tables comparing standard aerospace-grade fibers (T300, T700, T800, M40J, M60J) to aluminum, steel, and titanium on both an absolute and a specific (per-weight) basis.

The Fiber-Matrix Partnership: Why CFRP Properties Aren’t Just the Fiber’s Properties

A T700S carbon fiber filament has a tensile strength of 4,900 MPa. A T700S/epoxy unidirectional laminate tested in the fiber direction has a tensile strength of about 2,500-2,800 MPa—roughly half the fiber strength. The difference isn’t a measurement error. It’s the fiber volume fraction: a unidirectional laminate is typically 55-65% fiber by volume, with the remaining 35-45% being epoxy resin whose tensile strength is about 70-80 MPa. The rule of mixtures gives the composite’s longitudinal strength as roughly the fiber strength multiplied by the fiber volume fraction (plus a negligible contribution from the resin). If you want higher composite strength, you need higher fiber volume fraction—which is why autoclave-cured prepreg (60-65% FVF) outperforms wet layup (35-45% FVF) by roughly 30-40% at the same fiber type. But fiber volume fraction isn’t free. Higher FVF laminates become more brittle in the transverse direction because there’s less resin to distribute load between fibers. The practical ceiling for aerospace CFRP is about 65% FVF—beyond that, the laminate becomes too difficult to process and the transverse properties degrade to the point where interlaminar shear becomes the governing failure mode. The key takeaway: when comparing carbon fiber mechanical properties, always ask whether the number is for the fiber alone (the tow test) or for a specific laminate with a known fiber volume fraction and layup. A T300 fiber tested as a dry tow gives about 3,530 MPa. A T300/ epoxy quasi-isotropic laminate (0/+45/-45/90) gives about 600-700 MPa in any in-plane direction. Both are correct—they’re measuring different things.

Carbon Fiber vs Metals: The Specific Properties That Actually Matter

Comparing carbon fiber to metals on absolute strength is a category error. A unidirectional T700S/epoxy laminate at 60% FVF beats 7075-T6 aluminum in longitudinal tensile strength (2,550 MPa vs 572 MPa)—but loses badly in transverse tension (about 50 MPa vs 572 MPa). The fair comparison is specific properties: strength or stiffness divided by density, which tells you how much structural capability you get per kilogram. On specific tensile strength, CFRP (T700S, 60% FVF UD) achieves about 1,600 MPa·cm³/g. Aluminum 7075-T6 achieves about 203 MPa·cm³/g. CFRP wins by a factor of eight. On specific modulus, CFRP (T700S UD) achieves about 82 GPa·cm³/g versus aluminum’s 26 GPa·cm³/g—a factor of three. This is why carbon fiber dominates aerospace structures: when every kilogram saved is worth hundreds of dollars in fuel over the aircraft’s life, an eight-to-one specific strength advantage is a compelling business case that overrides the higher material cost. The comparison shifts when you move from unidirectional properties to quasi-isotropic laminate properties—which is what a real structural part uses, because real loads come from multiple directions. A quasi-isotropic T700S/epoxy laminate (0/±45/90 layup) has a tensile strength of about 650 MPa and a modulus of about 50 GPa in any in-plane direction. On absolute strength, that’s comparable to 7075-T6 aluminum (572 MPa yield). On specific strength, it’s about 420 MPa·cm³/g versus aluminum’s 203—still a factor-of-two advantage. The carbon fiber advantage survives the transition from unidirectional test coupon to quasi-isotropic structural laminate. It just drops from a factor of eight to a factor of two—which is still enough to save 20% on structural weight in a well-designed part.

Compression and Flexural Properties: Where CFRP Hits Its Limits

Carbon fiber’s compressive strength is systematically lower than its tensile strength—typically 60-70% of the tensile value for a unidirectional laminate. T700S UD laminate: tensile 2,550 MPa, compressive about 1,500 MPa. The failure mechanism is different: tensile failure is fiber-dominated (the fibers break), while compressive failure is matrix-dominated (the fibers buckle microscopically within the resin matrix, a phenomenon called microbuckling). Higher-modulus fibers (M40J, M60J) have worse compressive-to-tensile ratios because their smaller diameter and higher anisotropy make them more susceptible to microbuckling. This is why carbon fiber structures that see compressive loads—upper wing skins, compression-loaded struts—need to be designed with the compressive allowables front and center, not the tensile numbers that manufacturers put in the headline of the datasheet. Flexural strength sits between tensile and compressive—typically 1,200-1,700 MPa for aerospace-grade CFRP—because a bending test puts one surface in tension and the other in compression, and failure usually initiates on the compression side. The practical implication: if your part sees bending loads (beams, plates under pressure, shells), design to the compressive strength, because that’s where failure will start.

Fatigue: Where Carbon Fiber Embarrasses Metals

If there’s one mechanical property where carbon fiber’s advantage over metals is genuinely overwhelming, it’s fatigue. Aluminum has no fatigue limit—it will eventually fail under any cyclic stress, given enough cycles. Steel has a fatigue limit at about 40-50% of its tensile strength. Carbon fiber/epoxy composites tested in the fiber direction at 10⁷ cycles retain 70-90% of their static strength—and the S-N curve is essentially flat after about 10⁶ cycles, meaning there may be a practical fatigue limit at around 70-80% of static strength. In the Boeing 787’s composite fuselage, this fatigue advantage translates directly into lower maintenance costs: no lap-joint fatigue cracking inspections (the classic aluminum fuselage maintenance driver), no corrosion-related fatigue (because CFRP doesn’t corrode), and a design service life of 44,000 flight cycles versus 20,000-30,000 for aluminum-intensive designs. The catch is that fatigue in CFRP is matrix-driven: at high temperatures or in the presence of moisture, the resin’s fatigue resistance degrades, and the composite’s fatigue performance drops toward the resin’s fatigue limit. A CFRP part that retains 80% of its static strength after 10⁷ cycles at room temperature in dry conditions might retain only 50-60% at 80°C with moisture saturation. This is why aerospace CFRP structures are designed with environmental knockdown factors—a topic for another article, but the headline is: carbon fiber’s fatigue advantage is real, large, and temperature-dependent.